<html>
<head><title>Calculator</title></head>
<body>

<h1>Calculator</h1>
(By Matthew Berry) <br>
Type an expression in the following box (e.g., 10.5+20*3/25).
<p>
<form method="GET">
<input type="text" name="expr"><input type="submit" value="Calculate">
</form>
</p>
<ul>
<li>Only numbers and +,-,* and / operators are allowed in the expression.
<li>The evaluation follows the standard operator precedence.
<li>The calculator does not support parentheses.
<li>The calculator handles invalid input "gracefully". It does not output PHP error messages.
</ul>
Here are some(but not limit to) reasonable test cases:
<ol>
  <li> A basic arithmetic operation:  3+4*5=23 </li>
  <li> An expression with floating point or negative sign : -3.2+2*4-1/3 = 4.46666666667, 3+-2.1*2 = -1.2 </li>
  <li> Some typos inside operation (e.g. alphabetic letter): Invalid input expression 2d4+1 </li>
</ol>
<h2>Result</h2>

<?php

$equ = $_GET["expr"];

$regex1 = "#^[0-9+-][0-9*/+-]{0,}$#";	//should be true
$regex2 = "#[*/+-][*/]+#";  		//should be false
$regex3 = "#(/0)+#";			//should be false
$regex4 = "#(--)+#";

if (preg_match($regex1, $equ)==0 || preg_match($regex2, $equ)==1 || preg_match($regex3, $equ)==1 || preg_match($regex4, $equ)==1) {
    echo "Invalid input expression ".$equ;

} else {

eval("\$ans = $equ ;");
echo $equ." = ".$ans;

}


?>

</body>
</html>
